1. Field of the Invention
The present invention relates to an actuator applied to, e.g., the hood apparatus of a car.
2. Description of the Related Art
If a pedestrian were hit on the front of a running car, the lower half of his/her body may be stricken by the bumper at the nose of the car. In this case, he/she is thrown on the hood of the car and hit, e.g., the head against it.
Under these circumstances, some cars have hood apparatuses. A hood apparatus is designed to operate an actuator to push the hood upward when a sensor arranged at the nose of the car detects a pressure with a predetermined value or more. When the hood is pushed up, a gap is formed between the hood and the devices in the engine room. Even when a pedestrian jumps up and hits against the hood, the collision energy is absorbed by hood deformation.
A hood apparatus is known, which is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-29369. The actuator in this hood apparatus can cause the piston pin (piston rod) to project from the cylinder and drop again in the cylinder.
This actuator comprises a piston stop member which stops the raised piston pin to prevent it from leaving the cylinder unit, a lock piece which locks the piston pin stopped by the function of the piston stop member to prevent it from dropping, a compression spring which elastically pushes the lock piece to the cylinder unit, and an unlock means which returns the lock piece to a predetermined position to cancel the lock function of the lock piece. In this actuator, when the unlock means cancels the lock function of the lock piece, the piston pin drops. The unlock means is controlled by the control unit in the hood apparatus.
Another hood apparatus is known, which is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-115680. The actuator in this hood apparatus has an exhaust hole with a very small hole diameter at the butt portion between the actuator and the piston portion of the push rod (piston rod) of the cylinder unit in the normal state. The exhaust hole is closed by a sealing plug. A bracket is attached to the projecting portion of the connecting portion of the push rod from the cylinder unit. The bracket and sealing plug are connected by a wire. When the piston rod moves up, the sealing plug is pulled through the wire and removed from the exhaust hole. A high-pressure gas in the cylinder unit is pushed out from the exhaust hole so that the push rod drops at a predetermined speed.
However, the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-29369 has no arrangement which discharges the high-pressure gas from the cylinder unit. For this reason, even when the lock function of the lock piece is canceled by the unlock means, the piston pin may be pushed by the high-pressure gas and not satisfactorily drop in the cylinder unit.
On the other hand, in the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-115680, the high-pressure gas in the cylinder unit is discharged outside by forming the exhaust hole in the cylinder unit. In forming the exhaust hole in the cylinder unit, a burr may remain in the cylinder unit. The burr is not preferable because it increases the sliding resistance of the piston in the cylinder unit. In addition, the sealing plug that closes the exhaust hole may be removed from the exhaust hole due to the high-pressure gas before the push rod reaches the full-stroke position. If the sealing plug is removed from the exhaust hole before the push rod reaches the full-stroke position, the push rod may not satisfactorily project from the piston portion.